feat(props): make dim-line colour editable for leaders and dimensions

The "Dim line color" row on Leader and Dimension entities was
read-only: an earlier attempt wired it to Leader.override_color, which
acadrust never serialises, so the pick was lost on save. Store it
instead as a standard ACAD_DSTYLE per-object dimension-style override
(DXF code 176, an ACI index) through the existing dim_override codec,
so it round-trips through both DWG and DXF like the other dim
overrides. RGB picks collapse to the nearest ACI, matching the rest of
the dim-colour stack (dimension styles are index-only through the file
layer). The renderer prefers the override over the style's DIMCLRD.

The write branch is guarded to leaders and dimensions so a mixed
selection cannot stamp the override onto other entity types.

Bumps acadrust to c9fe982, which carries parsed XDATA onto dimensions
on DXF read (it was dropping common.extended_data), so the dimension
override persists on DXF save as well as DWG.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entities/dim_override.rs

@@ -6,6 +6,7 @@
 //! renderer and the properties panel prefer an override over the style default,
 //! so editing one of these rows writes here and the change round-trips to file.
 
+use acadrust::types::Color;
 use acadrust::xdata::{ExtendedData, XDataValue};
 use acadrust::{CadDocument, Handle};
 
@@ -13,6 +14,7 @@ use acadrust::{CadDocument, Handle};
 pub const DIMSCALE: i16 = 40; // overall scale       (real)
 pub const DIMASZ: i16 = 41; // arrow size          (real)
 pub const DIMTAD: i16 = 77; // text vertical pos   (int16)
+pub const DIMCLRD: i16 = 176; // dim line colour     (int16 = ACI index)
 pub const DIMGAP: i16 = 147; // text offset / gap   (real)
 pub const DIMLWD: i16 = 371; // dim line lineweight (int16)
 pub const DIMLDRBLK: i16 = 341; // leader arrow block  (handle)
@@ -58,6 +60,13 @@ pub fn int(xd: &ExtendedData, code: i16) -> Option<i16> {
         })
 }
 
+/// The colour override for `code`, if present. Dim-colour overrides are stored
+/// as an ACI index (the same `int16` slot the dimension style uses), so this
+/// decodes it back into a `Color` (0 = ByBlock, 256 = ByLayer, else indexed).
+pub fn color(xd: &ExtendedData, code: i16) -> Option<Color> {
+    int(xd, code).map(Color::from_index)
+}
+
 /// The handle-valued override for `code`, if present.
 pub fn handle(xd: &ExtendedData, code: i16) -> Option<Handle> {
     pairs(xd)